Fine-pore Diffusers Research Articles

Model-based monitoring of diffuser fouling using standard sensors

Fouling of fine-pore diffusers can cause substantial aeration energy wastage. It remains challenging to monitor their condition and decide the optimal time for labour-intensive and costly cleaning actions. In this study, we show that data from standard sensors (airflow rate, dissolved oxygen concentration, pressure and airflow valve position), which are fed to simple models, can track the diffuser's condition. Additionally, the parameter estimation of diffuser dynamic wet pressure, oxygen transfer rate, respiration rate and the joint alpha fouling factor (αF) was facilitated by an active fault detection inspired method. The method executes a sequence with piecewise constant valve positions via the control system. As a result, airflow rates in a sequence similar to a staircase are obtained, which simplifies the estimation of dissolved oxygen dynamics and airflow rate dynamics. The proposed method was evaluated on a full scale over 18 months and successfully detected a reduced cleaning in the diffusers and several sensor-related disturbances. Ultimately, the findings motivate further research on how modelling combined with repetitive process disturbances can leverage data-driven insights from standard instrumentation.

Impact of scaling on aeration performance of fine-pore membrane diffusers based on a pilot-scale study

Aeration systems consume a large amount of energy in wastewater treatment plants. Fine-pore membrane diffusers are most commonly used in aeration systems. Scaling and fouling on these membrane diffusers will lead to decreased performance in aeration and increase energy consumption. This pilot-scale study focused on the scaling of the three kinds of fine-pore membrane diffusers under different influent hardness conditions. The results showed that the diffusers were mainly polluted by calcium carbonate scaling. Scaling occurred on the outer surface, orifices and inner surface of the membranes. The dynamic wet pressure (DWP) of ethylene-propylenediene monomer (EPDM), silicone and polyurethane (PU) membrane diffusers increased by 126%, 34% and 304%, respectively, within 50 days when the hardness was 400 mg/L (as CaCO3). However, the increase ratio became obviously slow during the subsequent 60-day operation, indicating a scaling rule of membrane diffusers. Considering that the standard aeration efficiency (SAE) acted as a comprehensive index for judging the aeration performance, the silicone diffuser had better performance than the other two diffusers when severe scaling occurred. This research also provides basic support for the design of membrane diffusers to improve their anti-scaling performance.

Fouling characterization and aeration performance recovery of fine-pore diffusers operated for 10 years in a full-scale wastewater treatment plant

Fouling characterization and aeration performance recovery of fine-pore diffusers operated for 10 years in a full-scale wastewater treatment plant were investigated to elucidate fouling mechanisms and develop cleaning strategy. The performance decline of diffusers was observed with dynamic wet pressure increased by 3.2 times and standard oxygen transfer efficiency dropped to 73%, which contributed to 15.0% increase in total energy consumption. Oxygen-affinity, filamentous and extracellular polymeric substances secreting bacteria tended to accumulate on the diffuser surface. External (mainly biofilm growth), internal (organic and inorganic matters) and irrecoverable (mainly material aging) foulants accounted for 34.1%, 45.4% and 20.1% of total fouling, respectively. HCl cleaning failed to restore aeration efficiency because it eliminated structural support formed by inorganics, leaving organic foulants broken into smaller fragments and distributed more dispersed. NaClO showed better cleaning efficiency by effectively removing organic foulants. Sequential cleaning by NaClO and HCl, which achieved the best recovery, was recommended.

Review of the Factors Affecting Modeling Oxygen Transfer by Fine-Pore Diffusers in Activated Sludge.

In this review, the factors affecting the transfer of oxygen in activated sludge processes using fine-pore diffusers for water resource recovery are critically discussed. In water resource recovery facilities, the energy required for aeration constitutes 50% to 80% of the total energy consumed by the plant. This critical review highlights the use of fine-pore diffuser aeration and emphasizes the significance of accounting for the following factors: diffuser aging and fouling, diffuser layout, diffuser type, selector benefits, local environmental conditions (temperature and atmospheric pressure), influent wastewater variability, dissolved oxygen control systems, and airflow rates. In our review, we were unable to find mathematical models that could be used to develop dynamic alpha factor predictions and diffuser fouling predictions. Although the development of a model that considers all the factors that affect oxygen transfer efficiency (OTE) in activated sludge systems would be extremely valuable, the creation of such a model is outside the scope of this review. This article is protected by copyright. All rights reserved.

A Critical Review of the Factors Affecting Modeling Oxygen Transfer by Fine-Pore Diffusers in Activated Sludge.

In this review, the factors affecting the transfer of oxygen in activated sludge processes using fine-pore diffusers for water resource recovery are critically discussed. In water resource recovery facilities, the energy required for aeration constitutes 50% to 80% of the total energy consumed by the plant. This critical review highlights the use of fine-pore diffuser aeration and emphasizes the significance of accounting for the following factors: diffuser aging and fouling, diffuser layout, diffuser type, selector benefits, local environmental conditions (temperature and atmospheric pressure), influent wastewater variability, dissolved oxygen control systems, and airflow rates. In our review, we were unable to find mathematical models that could be used to develop dynamic α-factor predictions and diffuser fouling predictions. Although the development of a model that considers all the factors that affect oxygen transfer efficiency (OTE) in activated sludge systems would be extremely valuable, the creation of such a model is outside the scope of this review.

Effects of air bubble size on algal growth rate and lipid accumulation using fine-pore diffuser photobioreactors

Microalgae are a potential feedstock for biodiesel and aeration is a reliable method to improve the growth of microalgae. Previous studies revealed the influence of bubbles on a variety of microorganisms, yet so far few studies have focused on the effects of bubble size on microalgal lipid production. In this study, four different fine-pore diffusers that could produce different bubble sizes were used for aeration in the photobioreactors. The equivalent diameters (de) of the bubbles in the four systems were 1.5 mm, 2.5 mm, 3.5 mm and 6.0 mm, respectively. The system with de = 3.5 mm obtained the highest biomass productivity (104.1 mg·L−1·d−1), which was 12% higher than that with de = 1.5 mm. However, the lipid content in the system with de = 1.5 mm was 30% higher than that for de = 3.5 mm, and it also achieved the highest lipid productivity of 30.4 mg·L−1·d−1. Results from transmission electron microscope indicated that there is more starch in the cells when de = 3.5 mm and more lipid droplets in the cells when de = 1.5 mm. The largest bubble size was not beneficial for supplementing CO2 to the algae, which may have limited the growth. Therefore the air bubble size of de = 1.5 mm is the best when using aeration as a strategy for enhancing lipid productivity, since it could provide appropriate external stress for lipid accumulation, but did not greatly inhibit the growth of microalgae.

Removal mechanisms of volatile organic compounds (VOCs) from effluent of common effluent treatment plant (CETP)

This study investigated the occurrence, removal and influence of plant-operating conditions on removal mechanisms of 83 VOCs in different treatment units of a CETP in Mumbai, treating industrial waste on primary and secondary level. A mass balance approach was used to predict VOC removal by volatilization, stripping, weir drop, adsorption, and biodegradation. Results indicate that ∼17% of VOCs were removed by stripping in equalization tank and ∼8% were removed by weir drop in primary clari-flocculator respectively. Biodegradation was the dominant mechanism in aeration tank and was relatively poor for hydrophobic compounds which were more vulnerable to removal by stripping. Stripping rates could be reduced by increasing the active biomass concentration and using fine pore diffusers to reduce the air/effluent ratio. Decrease in Henry's constant and compound concentration can shift the main removal mechanism from stripping to biodegradation. Results also show considerable agreement between measured (71.2%) and predicted (67.1%) total removal, especially in aeration tanks. Equalization tanks (actual, 20.5%, predicted, 16.9%), primary clari-flocculator (actual, 14.2%, predicted, 7.7%), and secondary clarifier units (actual, 29.5%, predicted, 16.8%) showed fairly acceptable differences in measured and predicted removal. The effect of other mechanisms on VOC removal need to be further explored owing to their major contribution to VOC removal. This study is the first attempt in understanding the mechanisms behind the removal of VOCs in each treatment unit, especially equalization tanks and clarifier units, which have been severely underestimated till date.

Reverse flexing as a physical/mechanical treatment to mitigate fouling of fine bubble diffusers.

Achieving energy neutrality has shifted focus towards aeration system optimization, due to the high energy consumption of aeration processes in modern advanced wastewater treatment plants. A study on fine bubble diffuser fouling and mitigation, quantified by dynamic wet pressure (DWP), oxygen transfer efficiency and alpha was carried out in Blue Plains, Washington, DC. Four polyurethane fine bubble diffusers were installed in a pilot reactor column fed with high rate activated sludge from a full scale system. A mechanical cleaning method, reverse flexing (RF), was used to treat two diffusers (RF1, RF2), while two diffusers were kept as a control (i.e., no reverse flexing). There was a 45% increase in DWP of the control diffuser after 17 months of operation, an indication of fouling. RF treated diffusers (RF1 and RF2) did not show significant increase in DWP, and in comparison to the control diffuser prevented about 35% increase in DWP. Hence, reverse flexing potentially saves blower energy, by reducing the pressure burden on the air blower which increases blower energy requirement. However, no significant impact of the RF treatment in preventing a decrease in alpha-fouling (αF) of the fine pore diffusers, over time in operation was observed.

Determination of diffuser bubble size in computational fluid dynamics models to predict oxygen transfer in spiral roll aeration tanks

The aeration tank is one of the most important processes in the biological wastewater treatment and water reuse system to remove organic pollutants in wastewater. The prediction of oxygen transfer using the computational fluid dynamics (CFD) method is crucial for minimizing the energy consumption of the aeration tank, which is one of the highest energy consuming units in a wastewater treatment system. Bubble size (dB), which is a critical parameter in the CFD simulations, was investigated for different types of diffusers. CFD calculations were conducted to simulate the hydraulics in different aeration tanks and the calculated values of the volumetric oxygen mass transfer coefficient (KLa) were compared with experimentally measured data. A total of 19 tanks containing clean water and 8 tanks containing wastewater with different diffuser types and aeration intensities were simulated. The KLa values determined for these configurations were fitted using a calibrated dB value specified in the CFD simulations. The results of the calculations indicate that the coarse-bubble diffusers, fine-pore diffusers, and slitted membrane diffusers have bubble sizes of 7–8mm, 5–6mm, and approximately 3mm, respectively. Additionally, CFD simulations were conducted to simulate the flow pattern and calculate the corresponding KLa value when the diffuser configuration was changed.

Linking biofilm growth to fouling and aeration performance of fine-pore diffuser in activated sludge

Aeration is commonly identified as the largest contributor to process energy needs in the treatment of wastewater and therefore garners significant focus in reducing energy use. Fine-pore diffusers are the most common aeration system in municipal wastewater treatment. These diffusers are subject to fouling and scaling, resulting in loss in transfer efficiency as biofilms form and change material properties producing larger bubbles, hindering mass transfer and contributing to increased plant energy costs. This research establishes a direct correlation and apparent mechanistic link between biofilm DNA concentration and reduced aeration efficiency caused by biofilm fouling. Although the connection between biofilm growth and fouling has been implicit in discussions of diffuser fouling for many years, this research provides measured quantitative connection between the extent of biofouling and reduced diffuser efficiency. This was clearly established by studying systematically the deterioration of aeration diffusers efficiency during a 1.5 year period, concurrently with the microbiological study of the biofilm fouling in order to understand the major factors contributing to diffuser fouling. The six different diffuser technologies analyzed in this paper included four different materials which were ethylene-propylene-diene monomer (EPDM), polyurethane, silicone and ceramic. While all diffusers foul eventually, some novel materials exhibited fouling resistance. The material type played a major role in determining the biofilm characteristics (i.e., growth rate, composition, and microbial density) which directly affected the rate and intensity at what the diffusers were fouled, whereas diffuser geometry exerted little influence. Overall, a high correlation between the increase in biofilm DNA and the decrease in αF was evident (CV < 14.0 ± 2.0%). By linking bacterial growth with aeration efficiency, the research was able to show quantitatively the causal connection between bacterial fouling and energy wastage during aeration.

Framework for Energy Neutral Treatment for the 21st Century through Energy Efficient Aeration

Aeration is commonly identified as the most significant energy use in the treatment of wastewater and therefore garners significant focus in reducing energy use in the ultimate aspirational goal of achieving net energy neutrality for water resource recovery. This research establishes the mechanistic link between biofilm DNA concentration and reduced aeration efficiency caused by biofilm fouling. Although the connection between biofilm growth and fouling has been implicit in discussions of diffuser fouling for many years, this research provides directly measured evidence of the connection between biofouling and reduced diffuser efficiency. This was clearly established by studying systematically the evolution of aeration diffusers' efficiency over time, concurrently with the biological study of the biofilm fouling the diffusers in order to understand the major factors contributing to diffuser fouling. While all diffusers foul eventually, some novel materials exhibited fouling resistance. Fouling mitigation techniques, such as chemical cleaning and membrane reverse-flexing were studied. However, during the course of this study the research could not conclude that these techniques were always effective in curbing effects of fouling. By linking bacterial growth with aeration efficiency and calculating the aeration process energy requirements from the latter, the research was able to show quantitatively the causal connection between bacterial fouling and energy wastage during aeration. The methodology applied here can be extended to perform parallel tests during the initial phases of aeration design to yield site-specific data that increase the robustness of design. Fine-pore diffused aeration is the most commonly used aeration system for new activated sludge treatment processes due to its high oxygen transfer efficiency. However, fouling of finepore diffusers has the double-negative effect of greatly diminishing their oxygen transfer efficiency and increasing the back-pressure on the blower, which decreases its operating efficiency. Development of a fundamental understanding of the science of fouling and its impact on fine-pore diffusers facilitates the development of mitigation strategies and guidelines. Overall, application of the findings of this research will result in reduced energy usage at water resource recovery facilities (WRRFs) that will enable them to close the gap between energy use and energy generation to ultimately reach the goal of energy neutrality. This title belongs to WERF Research Report Series ISBN: 9781780406794 (eBook)

Effect of biological process rate on fouling of fine-pore diffusers

Effect of biological process rate on fouling of fine-pore diffusers

Methodology for In Situ Column Testing to Improve Accuracy during Design and Specification of Aeration Systems

When designing wastewater aeration systems, fine-pore diffusers are typically selected for their potential energy efficiency. Because aeration is one of the most energy-intensive unit operations during treatment, the accurate quantification of aeration performance parameters is crucial to minimize the process energy footprint. Oxygen transfer efficiency in process water is the key design parameter; however, performance indicators that account for fouling and increased pressure drop with time must be considered. Both fouling and pressure drop increase cause an increase in blower power requirements. This paper presents a new approach to improve this design procedure, without altering the technical structure of the classical approach. Although the administrative and bidding milestones are underway (i.e., in the first few months of the project), an independent aeration team can test candidate diffusers suitable for design in an in situ aeration column. Subsequently, an extended fouling test in the plant’s wastewater can be carried out within the design timing, providing the designers with site-specific aeration performance data. This methodology improves the accuracy of the design process and increases overall energy savings.

Fine-Pore Diffuser Studies in BNR Wastewater: Performance Variation and Effects on Energy Footprint

Fine-Pore Diffuser Studies in BNR Wastewater: Performance Variation and Effects on Energy FootprintFine-pore diffusers are the most common aeration system in municipal wastewater treatment plants. Fouling is inevitable and inexorable for all types of fine-pore diffusers, and is dependent on process layout, water quality, diffuser type, and time in operation, but independent of diffuser make and model. The decline in diffuser performance, resulting in increased aeration energy footprint is...Author(s)Diego RossoLu-Man JiangJoseph ZhongDaniel H. CollerAndrew KimMaani FirouzianMatthew K. JeungMiguel Anaya-SantiagoKeith GellermanDavid M. HaydenPaul PittCharles S. HockingSourceProceedings of the Water Environment FederationSubjectSession 4: Pumping and Aeration: Wastewater Treatment Plant's Biggest Energy UsesDocument typeConference PaperPublisherWater Environment FederationPrint publication date Jan, 2011ISSN1938-6478SICI1938-6478(20110101)2011:6L.237;1-DOI10.2175/193864711802836409Volume / Issue2011 / 6Content sourceEnergy ConferenceFirst / last page(s)237 - 248Copyright2011Word count226Subject keywordsAerationfine-pore diffusersenergywastewateractivated sludgefouling

Real‐Time Aeration Efficiency Monitoring in the Activated Sludge Process and Methods to Reduce Energy Consumption and Operating Costs

Aeration is the most energy intensive unit operation in municipal wastewater treatment, and fine-pore diffusers have been widely used to minimize power consumption. Unfortunately, fine-pore diffusers suffer from fouling and scaling problems, which cause a rapid decline in aeration performance and significant increase in power consumption. Diffusers must be cleaned periodically to reduce energy costs. The cleaning frequency of diffusers is site-specific and its effectiveness can be evaluated with oxygen transfer efficiency (OTE) testing. Off-gas testing is the best technique for measuring OTE in real-time. Fine-pore diffusers have low a factors that are further reduced at high loading rate. A time-series of off-gas measurements were conducted to demonstrate the value of real-time OTE data for developing energy-conserving operating strategies. The observations confirm the inverse correlation between OTE and airflow rate as well as the economic benefits of diffuser cleaning. In addition, mathematic models were applied to simulate the transient oxygen uptake rate (OUR) and show the impact of varying load on OTE and aeration cost, especially when faced with time-of-day power rates. Regular diffuser cleaning can reduce average power costs by 18% and various equalization alternatives can reduce power costs by 6 to 16%.

Real-Time Efficiency Monitoring for Wastewater Aeration Systems

Aeration is the most energy intensive unit operation in municipal wastewater treatment. To improve oxygen transfer rate, fine-pore diffusers have been wildly applied in aeration practice. However, during operation, this type of diffuser suffers from fouling and scaling problems, which cause a rapid decline in aeration performance and significant increase in energy consumption. Diffusers must be cleaned periodically to reduce energy costs. The cleaning frequency of diffusers is site-specific, and can be evaluated by oxygen transfer efficiency (OTE) measurements over time in operation. Off-gas testing is the only technique that directly measures oxygen transfer efficiency in real-time. This paper presents a time-series of off-gas measurements which demonstrate the value of implementing energy-conservation practices. Our results include the real-time prediction of plant load and alpha factors from off-gas testing, as well as the quantification of the increased energy costs caused by fouling. Our off-gas analyzer can be used to develop an aeration efficiency monitoring protocols, and an aeration feedback control system for blowers.

Oxygen Transfer in a Full‐Depth Biological Aerated Filter

The City of San Diego, California, evaluated the performance capabilities of biological aerated filters (BAFs) at the Point Loma Wastewater Treatment Plant. The City conducted a 1-year pilot-plant evaluation of BAF technology supplied by two BAF manufacturers. This paper reports on the first independent oxygen-transfer test of BAFs at full depth using the offgas method. The tests showed process-water oxygen-transfer efficiencies of 1.6 to 5.8%/m (0.5 to 1.8%/ft) and 3.9 to 7.9%/m (1.2 to 2.4%/ft) for the two different pilot plants, at their nominal design conditions. Mass balances using chemical oxygen demand and dissolved organic carbon corroborated the transfer rates. Rates are higher than expected from fine-pore diffusers for similar process conditions and depths and clean-water conditions for the same column and are mostly attributed to extended bubble retention time resulting from interactions with the media and biofilm.

Aeration of large-scale municipal wastewater treatment plants: state of the art

Aeration is the most energy-intensive operation in wastewater treatment, amounting to 45-75% of plant energy costs. Fine-pore diffusers are today almost ubiquitous in municipal wastewater aeration, due to their advantageous aeration efficiency (mass of oxygen transferred per unit energy required). Nevertheless, older municipal treatment facilities and many industrial treatment plants are still equipped with coarse-bubble or surface aerators. Fine-pore diffusers are subject to two major disadvantages: a) fouling, if not cleaned periodically; b) decrease in oxygen transfer efficiency caused by dissolved surfactants. Coarse-bubble and surface aerators are typically not subject to the traditional problems affecting fine-pore diffusers. Nonetheless, they achieve oxygen transfer at the expense of increased energy intensity. The increased biomass concentration associated with high mean cell retention time (MCRT) operations has a beneficial effect on aeration. Nutrient-removing selectors are able to further increase aeration efficiency, as they sorb and utilize the readily available substrate which otherwise would accumulate at bubble surfaces and dramatically decrease aeration efficiency. We summarise here our 30-year long experience in aeration research, and results obtained with clean- and process-water tests are used to show the beneficial effects of high MCRT operations, the beneficial effect of selectors, and the decline of aeration efficiency due to dissolved surfactants.

Membrane properties change in fine-pore aeration diffusers: Full-scale variations of transfer efficiency and headloss

Fine-pore diffusers are the most common aeration system in municipal wastewater treatment. Punched polymeric membranes are often used in fine-pore aeration due to their advantageous initial performance. These membranes are subject to fouling and scaling, resulting in increased headloss and reduced oxygen transfer efficiency, both contributing to increased plant energy costs. This paper describes and discusses the change in material properties for polymeric fine-pore diffusers, comparing new and used membranes. Three different diffuser technologies were tested and sample diffusers from two wastewater treatment facilities were analysed. The polymeric membranes analysed in this paper were composed of ethylene–propylene-diene monomer (EPDM), polyurethane, and silicon. Transfer efficiency is usually lower with longer times in operation, as older, dilated orifices produce larger bubbles, which are unfavourable to mass transfer. At the same time, headloss increases with time in operation, since membranes increase in rigidity and hardness, and fouling and scaling phenomena occur at the orifice opening. Change in polymer properties and laboratory test results correlate with the decrease in oxygen transfer efficiency.

Accelerated Ageing Procedures for Assessing Decline in Fine-Pore Diffusers Performance

Accelerated Ageing Procedures for Assessing Decline in Fine-Pore Diffusers Performance